The only weird thing is the random freezes I encounter. It's like the image is freezing, but the computer seems to continue working for a bit longer. iTunes, for example, would still play until the end of the current song, and then stop (or start repeating 3-4 seconds of the start of the next song).

The only option then is to switch the computer off

The logs don't show anything special as far as I can tell...
Crash for the logs below was at 16:02.